Air heater having by-pass to prevent cold-end corrosion



FIGJ

AIR HEATER HAVING BY-PASS TO PREVENT COLD-END CORROSION Filed March 29, 1965 2 Sheets-Sheet 1 INVENTOR. Arfhur M. Frendberg AT TORNEY July 13, 1965 FIG.2

Filed March 29, 1963 A. M. FRENDBERG 3,194,214

AIR HEATER HAVING BY-PASS TO PREVENT COLD-END CORROSION 2 Sheets-Sheet 2 'l. 1 a I!" I I W W "1W" El g I f g I IN 2 1! United States Patent 3,194,214 AIR IEEATER HAVING BY-PASS TO PREVENT CQLlD-END CORROSION Arthur M. Frendberg, Akron, Ohio, assignor to The Bahcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Mar. 29, 1963, Ser. No. 263,985 3 Claims. (Cl. 122-1) The present invention relates to the construction and operation of heat exchange apparatus of the type providing for the indirect transfer of heat from one fluid to another.

More particularly, it relates to air heaters wherein the.

heating medium may be, for example, the hot gaseous products of combustion from a boiler furnace, while the medium to be heated may be the air required for combustion of fuel within the furnace.

In the conventional form of air heater wherein heat is transferred from combustion gases to combustion air, separate gas and air flow passages are provided through which they are directed in indirect heat exchange relation, the transfer of heat from one fluid to the other taking place through metallic walls forming separate passages, which walls may be utilized as heat transfer elements in the form of spaced metal plates, or more generally, in the form of spaced metal tubes.

In an air heater wherein the heat exchange elements are provided by spaced metal tubes arranged in one or more banks, the heating fluid, such as hot combustion gases, may be directed, through the tubes and thus through one set of passages, and the air directed over the outer surfaces of the tubes, and thus through a second set of passages provided between and around the tubes. A counterflow relation of air flow to gas flow is ordinarily maintained throughout the passages of any one bank, or succession of banks, for securing the maximum rate of heat transfer from one fluid to the other. In the operation of such an air heater, it is known that those tube lengths, or portions thereof, that are adjacent the cold air inlet may be subject to fairly rapid corrosion unless the air heater is operated so as to maintain all portions of the tubes adjacent the air inlet zone at temperatures sufficiently high to inhibit the condensation of entrained vapors in the gases passing through the tubes. This condition is known in the art as cold-end corrosion.

The corrosive effect mentioned is generally the result of the condensation of water vapor carried by the heating gases flowing in contact with the metallic heat transfer surfaces, combined with the presence of gaseous constituents, such as oxides of sulphur resulting from combustion of the sulphur bearing fuel by which the gases are generated. The condensation is promoted when the metal of the tubes, for example, within the cold gas exit end, is subjected for extended periods to relatively low temperature atmospheres due to the continuous contact of the cold inlet air with the exterior surface of the heat transfer elements or tubes, while at the same time there is continuous contact of the cooled gases with the interior tube. Condensation of the aqueous vapor also maintains the low temperature surface in a moist condition which is conducive to the formation of acidic sulphur compounds and to the collection of corrosive, entrained dust particles on the heat transfer surfaces with the consequent decrease in heat transfer efficiency. When the heat transfer elements are tubular members of relatively small diameter through which particle carrying gases are directed, such members are liable to become partially or even completely plugged and their effectiveness as heat transfer elements thus nullified. A similar plugging effect may result when using close-spaced plates with narrow gas flow passages there-.

between as heat transfer elements.

Since practical considerations with respect to air pressure drop dictate the use of large air flow pasages, there is ever present the danger of maldistribution of air flowing into and through the heater. This may result in rather wide variations in metal temperatures since the inlet end of the air heater may be subjected to widely different air mass flows, which will aggravate the corrosion problem. Thus, the corrosion condition described above is generally a local condition, and the air inlet portion should preferably be designed recognizing that it is necessary to compensate for the unbalance in metal temperatures caused by the maldistribution of air flow. In other words, the air heater must be designed to accommodate the expected minimum unbalanced air temperature rather than the mean air temperature.

The present invention is therefore concerned with improvements in heat exchangers of the indirect heat transfer type arranged, for example, for heating combustion air by means of hot gases resulting from the combustion of sulphur bearing fuel. More specifically, the objects of the present invention are to eliminate the well known cold-end corrosion problem and to minimize the design compensation necessary to account for air flow maldistribution in tubulous or plate-type air heaters of the type discussed above.

To accomplish these results, I provide an air heater having upright gas conducting tubes arranged in horizontally successive rows. The tubes are contained in a conduit, which also provides an air inlet and an air outlet, between which air is passed laterally over the exterior surfaces of the tubes in heat transfer relation therewith. The tubes may be disposed in three horizontally successive banks, the first or by-pass bank being adjacent the air inlet, the third bank adjacent the air outlet and the second bank intermediate the first and third banks. A gas inlet conduit is provided to direct .a first stream of heating gas from the convection pass outlet of the associated vapor generator to the upper ends of the tubes of the third bank to flow downwardly therethrough. A second or by-pass stream of higher temperature heating gas is withdrawn from an intermediate point in the convection pass of the associated vapor generator and is passed through a gas conduit leading to the upper ends or inlets of the tubes of the first pass. This higher temperature gas, passing downwardly through the first bank of tubes precludes the possibility of corrosion in the tubes nearest the cold air inlet, since the higher gas temperature will produce a correspondingly higher tube metal temperature and thus maintain the combustion gases above their dew point. A common hopper, provided with suitable baffles, is disposed beneath the three tube banks and serves as a mixing chamber for the two gas streams, as well as for collecting solids entrained in the gases. The combined streams then flow upwardly through the second or intermediate tube bank and pass out of the air heater. The air flows laterally over the three tube banks in serial order corresponding to their numerical designations. In passing over the first or bypass bank, the air tends to become uniformly distributed with respect to the inlet flowarea by virtue of the air pressure drop experienced incident to its passage through the tube bank. Thus the air flow distribution at the entrance to the intermediate bank of tubes, which is disposed within the region of lowest gas temperatures, will also be more uniform.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a partially diagrammatic sectional side view of a vapor generating unit embodying the inventive air heater; and

FIG. 2 is a top view of the air heater taken along line 22 of FIG. 1.

In the drawings, while the invention has been illustrated in embodied in conjunction with a forced-flow,

once-through vapor generating and superheating unit, it should be understood that the invention may advantageously be utilized in conjunction with vapor generators of other known designs, and also with other types of equipment without departing from the spirit of the invention.

Referring to FIG. 1, the main portions of the unit illustrated include an upright furnace chamber lit of substantially rectangular horizontal cross-section defined by a front wall 11, a rear wall 12, and side walls 13, a roof 14-, and a floor l5, and having a gas outlet 16 at its upper end opening to a horizontally extending gas pass 17 of rectangular vertical cross-section formed by extensions of the furnace roof 14 and side walls 13 and a gas pass floor 18. The gas-tight boundary walls of the furnace chamber 16 are formed by fluid heating tubes with suitable insulation secured exterior thereto. The gas pass 17 communicates at its rear end with the upper end of an upright gas passage 19 of rectangular crosssection defined by a front wall 2%, a rear wall 21, side walls 22 and a roof portion 23. The fuel firing equipment consists of independently operable, horizontally extending cyclone type furnaces 24 lined by fluid heating tubes and disposed in opposite walls 11 and 12 at the lower portion of the furnace chamber 16 Each cyclone furnace is arranged to burn solid fuel at high rates of heat release and to separately discharge high temperature gaseous products of combustion and separated ash residue asa molten slag into the lower portion of the chamber 19 through openings in the corresponding boundary wall thereof. The floor is formed with a plurality of openings (not shown) for the discharge of molten slag theretlnough. The gas pass 17 is occupied by secondary superheater sections 2'7 and reheater sections 28 arranged in series with respect to gas flow. The gas passage 19 is occupied by primary superheater sections 30 and economizer sections 31 arranged in series with respect to gas flow. The economizer sections 31 comprise horizontally extending multiple-looped tubes arranged for feedwater flow in counterflow relation with the heating gases. A cavity 32 is provided between the economizer sections 31 and the primary superheater sections 30. The lower end of the gas passage 19 is formed with an ash hopper 33 and a gas outlet 3% which leads to the air heater to be described hereinafter. If desired,'the lower end of the gas passage 19 may be formed with an additional outlet (not shown) leading to a gas recirculation system (not shown) as shown, for example, in US. Patent No. 2,960,972.

The air heater 46 shown in FIGS. 1 and 2, is formed with a plurality of upright gas conducting tubes 41 arranged in horizontally successive rows. The flow area encompassing the tubes 41 is defined by the continuously extending air heater side walls 42 and tube sheets 43 into Which the tubesrdl are fitted, to form a passage containing the tubes 41, and through which the air to be heated passes. Thus, the air to be heated passes from the forced draft fan 44 through the air inlet duct 45 to the air inlet 46 of the air heater 44 It then flows laterally over the outside of the tubes 41 to the air outlet 47. The heatedair is conducted to the cyclone furnace windboxes 48 through the hot air duct 49.

' The air heater tubes 41 are divided into three separate, horizontally successive tube banks; the first bank 59 comprising tubes 51A being adjacent the air inlet as, the third bank 52 comprising tubes 41C being adjacent sears the air outlet 47, and the second bank 51 comprising tubes 418 being disposed intermediate the first and third banks 50 and 52 respectively. It should be noted that the first bank St) is considerably smaller than either the second bank 51 or the third bank 52. A cavity 53 is provided between the first and second banks 56 and 51, and another cavity 54 is provided between the second and third banks 51 and 52. An ash hopper 55 underlies the three tube banks 5% 51 and S2. The hopper 55, formed with downwardly sloping walls 55A, 55B and 55C, serves the dual purposes of collecting ash entrained in the gas stream and directing and combining the flow of flue gases as will be hereinafter described. Downwardly and inwardly projecting gas baflles 56 and 57, provided heneath the second tube bank 51, extend into the cavity formed by the hopper 55. The function of these bafiles will also be hereinafter described in conjunction with the description of the fiow of flue gas through the air heater 40.

The combustion products, after traveling upwardly through the furnace chamber it and horizontally through the gas pass 17, turn and proceed downwardly through the convection pass 19. The main stream of flue gas leaves the convection gas pass 19 through the gas outlet 34 and passes through the air heater 40 in a generally counterflow path with respect to the air flow. The main flue gas stream is directed into the upper ends of the tubes 41C of the third bank 52 by the gas inlet duct 60, which is provided with a suitable expansion joint 61. After passing downwardly through the tubes 41C of the third bank 52, the gas stream, deflected and directed by the inclined wall 55A of the ash hopper 55, passes around the bafile 56, and turns upwardly to pass through the tubes 41B of the second bank 51. It should be noted that the contour of the hopper 55 is such that ash in the gas stream will be deposited in the ash hopper as the gas stream makes the abrupt change in direction of flow.

A smaller stream of hotter flue gas, withdrawn from the cavity 32 of the convection pass 19 through the bypass ports 76, passes through the bypass outlet ducts 71, thence through the horizontally disposed by-pass gas collection duct 72 which overlies the air heater and is adjacent the convection pass 19. Suitable expansion joints '73 are provided in the by-pass outlet ducts '71. A pair of bypass gas suply ducts 77 are connected with the ends of the collection duct 72 and extend rearwardly over the air heater 4%), and then downwardly to connect with the ends of the transversely disposed by-pass gas inlet duct 74 which overlies the first tube bank 56. The supply ducts 77 are also fitted with suitable expansion joints 75. The by p'ass gas flows through the above described ductworkand then passes downwardly through the tubes 41A of the first bank 59. By passing the hot by-pass gas through the tubes MA nearest the cold air inlet 46, the usual problem of cold-end corrosion is substantially eliminated. After passing through the tubes 41A of the first bank 56 the by-pass gas is directed by the baffle 57 and the hopper wall 55C toward the bottom of the hopper 55. By the proper positioning of the baffles 56 and 57, the by-pass gas stream and the main flue gas stream are thoroughly mixed before passing upwardly through the tubes 41B of the second tube bank 51, from whence they are conducted by the gas outlet duct 80 to the dust collector 81 and ultimately to a discharge stack (not shown).

As previously mentioned, the cold-end corrosion problem is avoided by passing the cold incoming air first over the tubes 41A of the first tube bank 50, through which the hot by-pass gas is flowing. This particular arrangement is also advantageous in that the tubes 41A of the first bank 59 serve to uniformly distribute the flow of :air as it enters the second tube bank 51 of the air heater 40. The improved distribution of the air over the entire flow area reduces the variations in air temperature entering the second tube bank 51 and therefore allows the tube bank 51 to be designed for maximum utilization of the available heating surface. In addition, the above described arrangement afiords a compact construction and requires a minimum amount of connecting ductwork to accomplish the desired result.

As a further demonstration of the applicability of the above described air heater arrangement, the following are calculated full-load typical design temperatures for an air heater for a large vapor generating unit similar to that shown in FIG. 1, and which is to be constructed in the near future by the assignee of the present inventor. The temperature of the main flue gas stream entering the tubes 41C of the third tube bank 52 will be about 680 F., and the temperature of the combined streams leaving the second tube bank 51 of the air heater 40 will be about 300 F. The by-pass gas temperature will be about 870 F.; the temperature in this instance being dictated by the practical limitations of the carbon steel ductwork leading from the by-pass ports to the first tube bank 50. The design entering air temperature is 100 F., while the air temperature in the cavity 53 is expected to be about 165 F. The quantity of gas flowing through the by-pass ductwork will be about percent of the total fine gas flow leaving the air heater 40.

While not considered necessary for the design herein described, it'will be understood that dampers, for regulating and controlling the flow of gas in the by-pass, may be readily adapted in a known manner within the flue and/ or duct layout.

Although the invention has been herein described in terms of an air heater having tubular heat exchange elements, arranged in indirect heat transfer relationship, it should be recognized that an air heater having plate-type heat exchange elements arranged according to the inventive concept herein disclosed, could readily be constructed without departing from the spirit of the invention.

While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invent-ion, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

What is claimed is:

1. An air heater comprising a plurality of upright gas conducting tubes arranged in horizontally successive rows, each of said tubes being substantially straight and having open upper and lower ends, means forming a passageway containing said tubes and providing an air inlet and an air outlet, means for passing the air to be heated laterally through said passageway and over said tubes in heat transfer contact therewith, said tubes being divided into banks including a first bank adjacent said air inlet, a third bank adjacent said air outlet, and a second bank intermediate said first and third banks, first gas inlet conduit means communicating with the upper ends of the tubes of said third bank, means for directing a first stream of heating gas through said first gas inlet conduit means and into the upper ends of the tubes of said third bank for downward passage therethr-ough, second gas inlet conduit means communicating with the upper ends of the tubes of said first bank, means for directing a second stream of heating gas through said second gas inlet conduit means and into the upper ends of the tubes of the first bank for downward passage therethrough, means communicating with the lower ends of the tubes of said first and third banks for combining said first and second streams of heating gas and directing them into the lower ends of the tubes of said second bank for upward passage therethrough, and gas outlet conduit means arranged to direct said combined streams of heating gas away from the upper ends of the tubes of the second bank.

2. In a vapor generator including a convection pass,

means forming a flue gas outlet at the discharge end thereof, a plurality of tubular heat exchange sections disposed therein, means forming a cavity between two of said heat exchange sections, there being at least one of said heat exchange sections between said cavity and said discharge end, and means forming an outlet port from said cavity in the wall of said convection pass, an air heater comprising upright gas conducting tubes arranged in horizontally successive rows, means forming a passageway containing said tubes and providing an air inlet and an air outlet between which air is passed over said tubes in heat transfer contact therewith, said tubes being divided into banks including a first bank adjacent said air inlet, a third bank adjacent said air outlet and a second bank intermediate said first and third banks, gas inlet conduit means arranged to direct a first stream of heating gas from said outlet port into the tubes of said first bank for passage therethrough, separate gas inlet conduit means arranged to direct a second stream of heating gas from said outlet portion into the tubes of said first bank for passage therethrough, said second stream of heating gas having a higher temperature than said first stream of heating gas, means for combining said first and second streams of heating gas and directing them into the tubes of said second bank for passage therethrough, and gas outlet conduit means arranged to direct said combined streams of heating gas away from the tubes of said second bank.

3. In a vapor generator includin a convection pass, means forming a fine gas outlet at the discharge end thereof, a plurality of tubular heat exchange sect-ions disposed therein, means forming a cavity between two of said heat exchange sections, and means forming an outlet port from said cavity in the Wall of said convection pass, an air heater comprising upright gas conducting tubes arranged in horizontally successive rows, means tormin g a passageway containing said tubes and providing an air inlet and an air outlet between which air is passed laterally over said tubes in heat transfer contact therewith, said tubes being divided into horizontally successive banks including a first bank adjacent said air inlet, a third bank adjacent said air outlet and a second bank disposed intermediate said first and third banks, gas inlet conduit means arranged to direct a first stream of heating gas from i said flue gas outlet into the upper ends of the tubes of said third bank, said first stream of'heating gas passing downwardly therethrough, separate gas inlet conduit means arranged to direct a second stream of heating gas from said outlet port into the upper ends of the tubes of said first bank, said second stream of heating gas passing downwardly therethrough, said second stream of heating gas having a higher temperature than said first stream of heating gas, means disposed below said banks for combrmng said first and second streams of heating gas and directing them into the lower ends of the tubes of said second bank, the combined streams of heating gas passing upwardly therethrough, said last named means including an ash hopper and a baflie, and gas outlet conduit means arranged to direct said combined streams of heating gas away from the upper ends of the tubes of said second bank.

References Cited by the Examiner UNITED STATES PATENTS 1,914,604 1/30 Keenan, et al.

2,386,138 10/45 Artsay.

2,744,733 5/56 HOWCS -56 X 2,947,522 8/60 Keller 134 X 3,007,681 11/61 Keller 165 103 X PERCY L. PATRICK, Primary Examiner. CHARLES SUKALO, Examiner. 

1. AN AIR HEATER COMPRISING A PLURALITY OF UPRIGHT GAS CONDUCTING TUBES ARRANGED IN HORIZONTALLY SUCCESSIVE ROWS, EACH OF SAID TUBES BEING SUBSTANTIALLY STRAIGHT AND HAVING OPEN UPPER AND LOWER ENDS, MEANS FORMING A PASSAGEWAY CONTAINING SAID TUBES AND PROVIDING AN AIR INLET AND AN AIR OUTLET, MEANS FOR PASSING THE AIR TO BE HEATED LATERALLY THROUGH SAID PASSAGEWAY AND OVER SAID TUBES IN HEAT TRANSFER CONTACT THEREWITH, SAID TUBES BEING DIVIDED INTO BANKS INCLUDING A FIRST BANK ADJACENT SAID AIR INLET, A THIRD BANK ADJACENT SAID AIR OUTLET, AND A SECOND BANK INTERMEDIATE SAID FIRST AND THIRD BANKS, FIRST GAS INLET CONDUIT MEANS COMMUNICATING WITH THE UPPER ENDS OF THE TUBES OF SAID THIRD BANK, MEANS FOR DIRECTING A FIRST STEAM OF HEATING GAS THROUGH SAID FIRST GAS INLET CONDUIT MEANS AND INTO THE UPPER ENDS OF THE TUBES OF SAID THIRD BANK FOR DOWNWARD PASSAGE THERETHROUGH, SECOND GAS INLET CONDUIT MEANS COMMUNICATING WITH THE UPPER ENDS OF THE TUBES OF SAID FIRST BANK, MEANS FOR DIRECTING A SECOND STEAM OF HEATING GAS THROUGH SAID SECOND GAS INLET CONDUIT MEANS AND INTO THE UPPER ENDS OF THE TUBES OF THE FIRST BANK FOR DOWNWARD PASSAGE THERETHROUGH, MEANS COMMUNICATING WITH THE LOWER ENDS OF THE TUBES OF SAID FIRST AND THIRD BANKS FOR COMBINING SAID FIRST AND SECOND STREAMS OF HEATING GAS AND DIRECTING THEM INTO THE LOWER ENDS OF THE TUBES OF SAID SECOND BANK FOR UPWARD PASSAGE THERETHROUGH, AND GAS OUTLET CONDUIT MEANS ARRANGED TO DIRECT SAID COMBINED STREAMS OF HEATING GAS AWAY FROM THE UPPER ENDS OF THE TUBES OF THE SECOND BANK. 